In general, in fabricating semiconductor integrated circuits (semiconductor devices), substrates such as semiconductor wafers as object to be processed under treatment repetitively undergo predetermined procedures of film formation and pattern etching to build a number of desired devices.
During a variety of process steps applied to a object to be processed (substrate), the object, such as semiconductor wafer (hereinafter referred to as “wafer(s)”), had to be transported among processing units from one to another, and the wafers were unavoidably exposed to the atmospheric air during transport. Certain portions of wafer surfaces exposed to the atmospheric air (e.g., exposed portions of silicon substrates at the bottoms of contact-holes, or exposed portions of metal layers at the bottoms of through-holes) often got into reaction with oxygen and moisture in the air, and made native oxide films. There was also the possibility of producing chemical oxides on exposed surfaces due to reactions of such exposed portions with chemicals during wet cleaning (e.g., RCA cleaning). Additionally, there was another possibility that wafer surfaces were contaminated with substances such as metals during the transport of the wafers among the processing steps and among processing units.
Such oxides including native oxide films and chemical oxides (referred to as “native oxides” hereinafter) and metal contaminants degrade semiconductor properties such as electric properties, and thus, it has been usual to perform surface treatment to the wafer surfaces to remove the oxides and metal contaminants from them and clean them.
A typical technique conventionally employed as surface treatment to remove native oxide films and other undesired substances was wet cleaning (e.g. RCA cleaning) which immersed wafers in a liquid chemical such as HF solution to remove native oxide films and others. However, along with progressively increased density of integration and miniaturization of semiconductor devices, their dimensions including line widths and diameters of contact-holes are getting more and more miniaturized, which results in, for example, increasing aspect ratios of contact-holes and reducing their diameters to an extent around 0.2 to 0.3 μm or even smaller (e.g., 0.12 μm). Miniaturization to that extent caused the problems that liquid chemicals failed to sufficiently impregnate minute contact-holes, or on the contrary, liquid chemicals in the contact-holes could not go out therefrom due to their surface tensions. These problems sometimes led to the fatal disadvantage that native oxide films developed at the bottoms of the contact-holes could not be sufficiently removed.
In treating a multi-layered structure through the wet cleaning, since its respective layers exposed along walls of contact-holes made therethrough were different in etching rate, it caused further problems, such as irregularities in level of the wall surfaces of the contact-holes.
FIGS. 8A and 8B of attached drawings illustrate a contact-hole 202 for making electrical contact to a drain and a source formed on a surface of a wafer W of silicon (Si), for example. Hole diameter D shown in FIG. 8A is in the range from 0.2 to 0.3 μm, approximately. Multi-layered wall surfaces of the hole 202 are defined by silicon oxide films (SiO2) of three layers, for example, which are stacked in different film formation steps. For example, the first SiO2 film 204 is formed by thermal oxidization on the surface of the wafer W, the second SiO2 film 206 is made of phosphor-doped glass by spin coating, and the third SiO2 film 208 is made of silica glass. In addition to that, as shown in FIG. 8A, a native oxide film 210 is produced at the bottom of the contact-hole 202.
In such a three-layer-stacked layer, the SiO2 films 204, 206 and 208 are different in etching rate relative to a liquid chemical during wet cleaning. After the native oxide film 210 is removed by the wet cleaning, as shown in FIG. 8B, irregularities 209 are produced due to differences in etching rate, or border areas between adjacent layers where the liquid chemical can seep more easily are excessively eroded (see notches in the drawing). This is also a problem involved in the conventional wet-cleaning technique.
To overcome the above-mentioned disadvantages in the existing wet-cleaning technique, various alternative methods called dry-cleaning techniques (etching techniques) have been proposed as a replacement for wet-cleaning techniques relying on liquid chemicals, which used an etching gas to remove native oxide films (see, for example, Japanese Patent Laid-Open Publication No. hei 4-206526 and Japanese Patent Laid-Open Publication No. hei 6-196455).
Typically used is sputter etching with argon gas and H2 gas a method of removing native oxide films by dry cleaning.
In a method for burying through holes of semiconductor wafers with a metal as disclosed in Japanese Patent Laid-Open Publication No. hei 4-206526 referred to above, native oxide films or others are removed by pre-treating base metals partially exposed in a preliminary processing chamber, especially treating oxidized films overlying the metal films by supplying and heating ClF3 gas. Then that method transports the pre-treated wafers from the preliminary processing chamber to a film deposition chamber by a transport means without exposing them to the atmospheric air to conduct selective CVD of metals.
In the method of processing the wafers as disclosed in the above-identified Japanese Patent Laid-Open Publication No. hei 6-196455, wafers are placed in an atmosphere of a mixed gas of ClF3 and H2, and ultraviolet rays are irradiated to the mixed gas to thereby remove native oxide films produced on the wafers without heating the wafers.
However, the prior art sputter etching techniques relying on argon gas and hydrogen gas had the possibility of damaging contacts in the wafers, and there was the demand for a dry-cleaning technique using a low energy.
Additionally, the known cleaning technique using ClF3 gas involved the following problems:
That is, there was the problem that chlorine, derived from the ClF3 gas used for the cleaning, corroded metal films and others on wafers, and the resultant products of the semiconductor devices were degraded in yield and reliability. Since ClF3 gas is a chlorine-contained gas, after wafer surfaces were cleaned by using ClF3 gas, chlorine remained on the wafers in form of chlorine atoms combined with silicon or metals existing on wafer surfaces, for example, and the residual chlorine corrode metal films and others (as wirings of semiconductor devices, for example) formed on the wafers. Therefore, it invited deterioration of electrical properties of semiconductor, and degraded the reliability and yield of semiconductor devices as finished products.
Furthermore, there was the problem that reaction by the ClF3 gas excessively progressed, inviting damages to wafers and degradation of the yield and reliability of semiconductor devices as finished products. More specifically, after wafer surfaces were cleaned by using ClF3 gas, not only native oxides but also insulating films of SiO2 and metal films of Al intentionally formed on the wafers were undesirably etched by reaction with the ClF3 gas. When insulating films serving as interlayer insulating films in semiconductor devices and metal films serving as wiring in semiconductor devices are excessively etched, the resultant semiconductor devices degrade in electrical properties, and this inevitably results in decreasing the reliability and yield of semiconductor devices as final products.
The present invention is directed to overcoming the above-mentioned problems involved in the conventional cleaning technique for object surfaces to be processed, and it is an object of the present invention to provide a surface processing method and an apparatus therefor, which can improve the reliability of final products when used in process steps of manufacturing semiconductor devices.